298 THE GERM-LAYERS AND EVOLUTION 



a lengthy series of monographs these workers made a 

 comparative study of the mode of formation of the middle 

 layer, and arrived at a coherent theory of its origin. They 

 distinguished in the middle layer two quite distinct elements, 

 the mesoblast proper, formed by the evagination of the 

 walls of the archenteron, and the mesenchyme, formed by 

 free cells budded off from the germ-layers. The follow- 

 ing passage gives a good idea of their views and of 

 the phylogenetic implications involved : " Ectoblast and 

 entoblast are the two primary germ-layers which arise from 

 the invagination of the blastula ; they are always the first 

 to be laid down, and they can be directly referred back to 

 a simple ancestral form, the Gastraea ; they form the limits 

 of the organism towards the exterior and towards the arch- 

 enteron. The parietal and visceral mesoblast, or the two 

 middle layers, are always of later origin, and arise through 

 evagination or plaiting of the entoblast, the remainder of 

 which can now be distinguished as secondary entoblast 

 from the primary. They form the walls of a new cavity, the 

 enteroccel, which is to be regarded as a nipped-offdiverticulum 

 of the archenteron. Just as the two-layered animals can be 

 derived from the Gastraea, so can the four-layered animals 

 be derived from a Cctlom form. Embryonic cells, which 

 become singly detached from their epitheliar connections 

 we consider to be something quite different from the germ- 

 layers, and accordingly we call them by the special name of 

 mesenchyme germs or primary cells of the mesenchyme. 

 They may develop both in two-layered and in four-layered 

 animals. Their function is to form between the epithelial 

 limiting layers a secreted tissue (Secretgewebf) or connective 

 tissue with scattered cells, which cells can undergo, like the 

 epithelial elements, the most varied modifications. . . . This 

 secreted tissue in its simple or in its differentiated state, 

 with all its derivatives, we call the mesenchyme " (p. 122). 



The important point for us is that, just as all Metazoa 

 were considered by I laeckel to be descended from the Gastnca, 

 so all Cu-'lomati were held by the Hertwigs to be derived from 

 an original caelomate Crforni. In both cases an embryo- 

 logical archetype becomes a hypothetical ancestral form. 



The CcL'lom theory was considerably modified, extended 



